The present invention relates to a drive circuit of an active matrix type display device which is composed of thin-film transistors. In particular, the invention relates to a drive circuit of an active matrix type display device in which source followers are used as analog buffers and variations of their characteristics are suppressed.
The active matrix type display device is a display device in which pixels are arranged at intersections of a matrix with every pixel associated with a switching element, and image information is controlled by turning on/off of the switching elements. This type of display device uses, as a display medium, a liquid crystal, plasma, or some other material or state whose optical characteristic (reflectance, refractive index, transmittance, luminous intensity, or the like) can be varied electrically. In the present invention, specifically a field-effect transistor (three-terminal element) having the gate, source and drain is used as the switching element.
In the following description of the invention, a row of a matrix means a structure in which a signal line (gate line) that is disposed parallel with the row concerned is connected to the gate electrodes of transistors of the row concerned. A column means a structure in which a signal line (source line) that is disposed parallel with the column concerned is connected to the source (or drain) electrodes of transistors of the column concerned. A circuit for driving the gate lines is called a gate drive circuit, and a circuit for driving the source lines is called a source drive circuit.
In the gate drive circuit, stages of a shift register corresponding to the number of gate lines in the vertical direction are arranged linearly and interconnected in series to generate signals of vertical scanning timings of the active matrix type display device. In this manner, the thin-film transistors of the active matrix type display device are switched by means of the gate drive circuit.
In the source drive circuit, stages of a shift register corresponding to the number of source lines in the horizontal direction are arranged linearly and interconnected in series to generate horizontal image data of display image data of the active matrix display device. Analog switches are turned on/off by latch pulses that are synchronized with horizontal scanning signals. In this manner, currents are supplied to the thin-film transistors of the active matrix type display device by means of the source drive circuit, to control orientations of liquid crystal cells.
FIG. 9 schematically shows a conventional active matrix type display device. There are two kinds of polycrystalline silicon thin-film transistor manufacturing processes: a high-temperature process and a low-temperature process. In the high-temperature process, polycrystalline silicon is deposited on an insulating film that is formed on a quartz substrate, and a thermally oxidized SiO.sub.2 is formed as a gate insulating film. Thereafter, gate electrodes are formed, N-type or P-type ions are implanted, and source and drain electrodes are formed. Thus, polycrystalline silicon thin-film transistors are manufactured.
In the low-temperature process, silicon is crystallized by two kinds of methods: solid-phase growth and laser annealing. In the solid-phase growth, a polycrystalline silicon film is obtained by subjecting an amorphous silicon film on an insulating film that is formed on a glass substrate to a heat treatment of 600.degree. C. and 20 hours, for example. In the laser annealing, a polycrystalline silicon film is obtained by applying laser light to amorphous silicon on a glass substrate surface to thereby heat-treat only the film surface portion at a high temperature.
In general, crystalline films are obtained by using one or both of the above two methods.
An SiO.sub.2 film is then formed as a gate insulating film by plasma CVD. Thereafter, gate electrodes are formed, N-type or P-type ions are implanted, and source and drain electrodes are formed. Thus, polycrystalline silicon thin-film transistors are manufactured.
The source drive circuit is a circuit for supplying image data to an active matrix panel of the active matrix type display device by scanning it vertically, and is composed of a shift register, analog switches that are thin-film transistors, analog memories that are capacitors, and analog buffers formed of thin-film transistors.
The analog buffer is needed because the analog memory cannot directly drive the thin-film transistors of the active matrix type display device due to a large load capacitance of the source line.
The thin-film transistor of the analog buffer has a source follower configuration. As shown in FIGS. 6A and 6B, a single thin-film transistor is provided for each data holding control signal line, and the thin-film transistors are so manufactured as to be arranged at regular intervals.
FIG. 6A shows an example of using N-channel thin-film transistors. Alternatively, P-channel thin-film transistors (see FIG. 6b) or both types of transistors may be used.
The analog buffers that constitute the source drive circuit of the conventional active matrix type display device have the following problem.
Each analog buffer has the single thin-film transistor that has a source follower configuration. When laser annealing is employed as a means for crystallization as described above in the thin-film transistor manufacturing process, a silicon film on a glass substrate is irradiated with band-like laser light of a width L while being scanned with it in the X-axis direction, i.e., horizontally (see FIG. 7A) to crystallize silicon, because there exists no such large-diameter laser device as can irradiate a large-size substrate at one time.
When the illumination is effected while the laser light is moved in the X-direction at a constant length at a time, there occurs an overlap of illumination. Since the width L of the band-like laser light does not necessarily coincide with a pitch d (see FIG. 7B) of the source follower, the illumination laser light quantity varies depending on the position on the silicon film in the laser crystallization step.
Therefore, a positional variation, i.e., variations in characteristics occur in thin-film transistors that are produced from the above silicon film, and the threshold voltage V.sub.th varies from one thin-film transistor to another in the range of V.sub.thL to V.sub.thH depending on the position X on the X-axis (see FIG. 8). The threshold voltage V.sub.th has a small value at a position where laser beams overlap with each other, and has a large value where they do not. As a result, there occurs a variation in magnitude of output voltages of the source followers, which directly results in a variation of application voltages to the liquid crystal device.
FIG. 11 shows an application voltage vs. transmittance characteristic of a normally-white liquid crystal device. It is understood that a variation .DELTA.V.sub.th of the threshold voltage V.sub.th causes a corresponding variation of the transmittance, which is reflected in a displayed image.
As described above, the output voltages of the source drive circuit undesirably vary depending on positions thereof, resulting in display unevenness of pixels of the active matrix type display device.